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previous: Eric H [QUOTE]Thanks for your reply Eric; ... -- 12/27/2000 7:49 AM View Thread

Re: 1N34a diodes as clippers in a TS9?..

12/28/2000 3:54 AM
Mark Hammer
Re: 1N34a diodes as clippers in a TS9?..
Eric's correct. You have to remember that diodes clip by effectively setting a ceiling beyond which the signal level can't extend. If there is no additional gain stage after the clipping stage, then the actual maximum output will be determined by the clipping threshold of the diodes used.  
Depending on the gain and gain structure of the device, the type and number of diodes used for clipping will vary. If the gain of the device is high enough before the signal hits the diodes, you may need 2 pair of diodes in series to set an appropriate clipping point. If the gain is low enough before the diodes, you probably want diodes with a lower threshold to insure that some clipping takes place. The general principle to observe is: how often is my average signal level likely to exceed the clipping threshold? If it only rarely exceeds the threshold dictated by the diodes, then you may choose to use diodes with a lower threshold, OR build in more gain to the earlier stages of the pedal, OR stick some kind of booster ahead of the distortion pedal.  
Another thing to consider is that the actual clipping threshold will vary because of variations/tolerances in diodes. A pair of Ge diodes will generally clip at anywhere from 190mv to as high as 300mv of signal amplitude. A pair of Si diodes will clip at anywhere from about 400mv to 700mv, depending on the quirks of the actual diodes used. If you use a pair of diodes where the clipping threshold of one of them is different from the other, then you get asymmetrical clipping to varying degrees. For instance, combine two Si diodes where one has a 400mv clipping point and the other has a 700mv clipping point and one half cycle of the input signal will clip more readily than the other. The actual clipping threshold (which will determine the typical output level) can be tweaked by varying number and type of diodes. A 1N914 and 1N34 in series, paralleled with a similar reverse polarity pair, will likely get you a clipping threshold somewhere between about 650mv to 800mv, based on typical component values for each type of diode. A double pair of 1N34's will get you a clipping threshold between about 450-550mv, based on typically observed values.  
In principal, Ge diodes are supposed to produce a different quality of distortion from Si diodes, but in practice it is very hard to differentiate the tonal differences stemming from lower threshold from those coming out of the composition of the diode itself. I have a switch in my MXR Distortion+ clone that selects between a pair of Ge's and a pair of Si's. Flick the switch and the volume changes as well as the tone. Different clipping threshold, different amount of clipping and different ceiling on the output level.  
A fairer comparison would be to measure the cipping thrshold of the diodes you started out with, and select a combination if Ge diodes that produce the same clipping point. If your meter allows you to test diodes, you can determine the clipping threshold of combinations by simply adding up the threshold readings for each of the diodes in series. The higher the threshold, the less often the input signal will be sufficiently high to clip.